Fuel cells are classified into various types according to the type of electrolyte and the type of electrode, and as typical fuel cells, there are fuel cells of alkali type, phosphoric acid type, molten carbonate type, solid electrolyte type and solid polymer type. Of these, the solid polymer type fuel cells capable of working at a temperature of a low temperature (about −40° C.) to about 120° C. have been paid attention, and in recent years, development and practical use of them as low pollution power sources for automobiles have been promoted. As uses of the solid polymer type fuel cells, vehicle driving sources or stationary electric sources have been studied, but in order to apply the fuel cells to these uses, durability over a long term is desired.
This polymer solid type fuel cell is a fuel cell of such a type that a polymer solid electrolyte is interposed between an anode and a cathode, a fuel is supplied to the anode, oxygen or air is supplied to the cathode, and oxygen is reduced in the cathode to take out electricity. As the fuel, hydrogen, methanol or the like is mainly used.
In order to raise the reaction rate of the fuel cell and to enhance energy conversion efficiency of the fuel cell, a layer containing a catalyst (also referred to as a “catalyst layer for a fuel cell” hereinafter) has been provided on a surface of the cathode (air electrode) or a surface of the anode (fuel electrode) of the fuel cell in the past.
As this catalyst, a precious metal has been generally used, and of precious metals, platinum that is stable at a high potential and has high activity has been mainly used. However, since platinum is high in price and is limited on the resource quantity, development of catalysts capable of substitution has been desired.
Further, precious metals used for the cathode surface sometimes dissolve in an acidic atmosphere, and there is a problem that they are not suitable for uses requiring durability over a long term. On this account, development of catalysts which are not corroded in an acidic atmosphere, are excellent in durability and have high oxygen reduction ability has been eagerly desired.
As substitute catalysts for platinum, materials containing nonmetals, such as carbon, nitrogen and boron, have been paid attention in recent years. The materials containing nonmetals are low in price and rich in the resource quantity as compared with precious metals such as platinum.
In a non-patent document 1, it is reported that a ZrOxN compound containing zirconium as a base exhibits oxygen reduction ability.
In a patent document 1, an oxygen reduction electrode material containing a nitride of one or more elements selected from elements of Group 4, Group 5 and Group 14 of the long-form periodic table is disclosed as a substitute material for platinum.
In the materials containing these nonmetals, however, there is a problem that practically sufficient oxygen reduction ability as a catalyst has not been obtained.
In a patent document 2, an oxycarbonitride obtained by mixing a carbide, an oxide and a nitride and then heat treating the mixture at 500 to 1500° C. under vacuum or in an inert or non-oxidizing atmosphere is disclosed.
The oxycarbonitride disclosed in the patent document 2, however, is a material for a thin film magnetic head ceramic substrate, and it has not been studied to use this oxycarbonitride as a catalyst.
Platinum is useful not only as the above catalyst for a fuel cell but also as a catalyst for exhaust gas treatment or a catalyst for organic synthesis, but platinum is high in price and limited on the resource quantity, so that development of catalysts capable of substitution has been desired also in these uses.
Non-patent document 1: S. Doi, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, Journal of The Electrochemical Society, 154 (3) B362-B369 (2007)
Patent document 1: JP-A 2007-31781
Patent document 2: JP-A 2003-342058